Okay if you are going to max fire, than i suggest a rather "heavy" hybrid. Meaning not 1:7 not 1:5 but a little more hardcore. 80:80 is probably the best to go, this is 80 phys : 80 mag balance. There is a more specific guide on this build in the guide section of course
The reason why people take lightning over fire is because of the magic dmg increase buff. So if you take fire as a pure int instead of lightning than the damage gained from using the fire imbue + fire nuke will actually do less dmg than a lightning imbue + nuke (i dont know if that made sense xD). Also nukers aren't the best tankers so mobility is important.
